The objective of this study was to compare ultrasonography and low field MR imaging for evaluation of equine tendon injuries over a 16 week period. The sample population was the superficial digital flexure tendon of eight young adult horses. The percentage of experimentally-induced tendon injury was evaluated in cross-section at the maximal area of injury using both ultrasonography and MR imaging at 3, 4, 6, 8, and 16 week time points post collagenase injection. MR signal intensities (T1-weighted, T2-weighted, and STIR sequences) and tendon histology (collagen content, proteoglycan content, and collagen fiber alignment) was performed at the same time points. At four weeks following collagenase injury, the area of maximal injury assessed on cross-section was similar between ultrasonographic and MR imaging. In older lesions, ultrasonography significantly (P<0.001) underestimated the area of maximal cross-sectional injury by approximately 18 % when compared to MR imaging. Signal intensity of lesions on T1-weighted images was the most hyperintense of all sequences, with lesions on STIR images being slightly less hyperintense, and the T2-weighted images the most hypointense. Tendon lesion intensity was significantly higher than the normal deep digital flexor tendon. Histologically there was a decrease in proteoglycan content, an increase in collagen content, and minimal change in fiber alignment during the 16 weeks of the study. Ultrasonography may be underestimating the extent of tendon damage in chronic cases of tendinitis. Low field magnetic resonance provides a more sensitive way to evaluate tendon injury, and should be considered in cases of tendinitis of greater than 4 weeks duration.